1. Field of the Invention
This invention relates to thermal-magnetic trip units for circuit breakers and to circuit breakers incorporating such trip units. More particularly, it relates to an arrangement for preventing deformation of a bimetal providing the thermal trip function by the magnetic repulsion forces generated by short circuit currents.
2. Background Information
Circuit breakers typically provide protection against persistent overcurrent conditions and against the very high currents produced by short circuits. This type of protection is provided in many circuit breakers by a thermal-magnetic trip unit. The thermal portion of the trip unit is commonly a bimetal which is heated as a function of the magnitude and duration of the overcurrent. This causes the bimetal to bend and release the latch of a spring powered operating mechanism which opens the circuit breaker contacts to interrupt current flow. The very high current of a short circuit generates a magnetic field which acts upon an armature in the magnetic portion of the trip unit to unlatch the spring loaded operating mechanism.
In a common type of molded case circuit breaker in which the power contacts, operating mechanism and trip unit are mounted inside of a molded insulative housing, the bimetal is fixed to a cantilevered end of the load terminal conductor with the bimetal spaced from, but extending parallel to, the load terminal conductor. In a directly heated bimetal, a flexible shunt is connected adjacent the free end of the bimetal so that the bimetal and the load conductor form a folded electrical path between the flexible shunt and the load terminal in which current flows in one direction in the bimetal and in the opposite direction in the load terminal conductor. This generates magnetic repulsion forces which, in the case of the high currents associated with a short circuit, can be large enough to cause deformation of the bimetal. Some such terminal trip units have a calibration lever also fixed to the cantilevered end of the load conductor and positioned between the load conductor and the bimetal. A calibration screw threaded into an aperture in the load conductor engages the free end of the calibration lever. The calibration screw is turned to draw the free end of the calibration lever closer to or move it further away from the aperture in the load conductor. While the load conductor is made of copper, the calibration lever is typically made of steel so that rotation of the calibration screw results in bending of the cantilevered end of the load conductor. This results in adjustment of the free end of the bimetal relative to a trip bar to adjust the conditions under which the circuit breaker is tripped. As the calibration lever is made of steel, it provides some magnetic shielding for the bimetal, but only adjacent the fixed end of the bimetal.
There is a need for an improved thermal trip unit, and a circuit breaker incorporating the same, in which the large magnetic forces generated by short circuit currents do not result in deformation of the thermal trip bimetal.
There is a need for such an improved trip unit which does not require redesigning the basic structure of the trip unit.
There is an additional need for such an improved trip unit in which the protection against deformation of the bimetal requires minimal space.
There is yet another need for such an improved trip unit which is inexpensive and easy to implement.